Fix .btn-with-count documentation

[neildaniels]

The documentation incorrectly says that .with-count can be added to a .btn-sm. This doesn't work, but the example is working with .btn-with-count.

This changes the documentation to match the example.

/cc @primer/ds-core

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[joelhawksley]

Thanks for the PR @neildaniels!

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🦋 Changeset detected

Latest commit: 872431d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@primer/css	Patch

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